Powered blocks for hauling nets from the ocean to the deck of a vessel are hung from booms above the deck. The blocks are powered by hydraulic motors, requiring that hydraulic lines be strung up the boom and down to the block. Some blocks employ tailing wheels to press the net into the bight of the sheave. These blocks can only be run in one direction. As a consequence when changing the side of the ship from which the haul is being made, the block must be swiveled sometimes through more than 180.degree.. If only two or three hydraulic lines run to the block, keeping the lines from tangling or kinking is manageable. However, modern blocks with tailing wheels many times have up to seven lines running to the block, resulting in the need for a hydraulic swivel that allows the block to rotate without regard to twisting or kinking of lines.
While prior swivels may be used, all have certain drawbacks. First, they are all expensive and require high pressure seals. High pressure seals are responsible for high frictional faces in the swivel, thus causing undesirable resistance to rotation. Conventional swivels are made of steel and are therefore heavy and bulky. In addition, conventional swivels result in relatively high pressure drops, as the hydraulic fluid traverses the swivel, thus wasting energy.